[nfsv4] delegations are useful??

[Rick Macklem <[e-mail]>]

Hi,

For a very long time I have been a sceptic when it comes to whether
or not delegations are worth the complexity or effort of implementation.
Part of this is because, long ago, I implemented a caching mechanism
using delegation (NFSv4.0) and non-volatile storage and the results
could best be described as "not very satisfying".

I finally got around to implementing more of the NFSv4.1 (RFC8881)
file delegation semantics (in particular the atomic upgrade from read
to write delegation) and...
Running a simple test (which is a FreeBSD kernel build from sources
on a NFSv4.2 mount (sources and object on the mount), I see:
(Note that, starting in the 3rd column, they are RPC counts, where the
column name refers to the main operation in the RPC.)

Config               Elapsed time (sec) Getattr    Lookup   Access
Open/nocreate Open/create
no delegations  16876                     2341147  495422  225608
1112395           28224
file delegations  12734                      216570  445998  185045
 29056            28224
file and directory12537                     196549   298530  184503
 29056            28224
  delegations

As you can see, the file delegations have reduced RPC counts significantly. The
reduction on Open/nocreate was expected, but I'll admit the reduction
in Getattr RPCs
was not. Directory delegations reduced Lookup RPCs by about 30%, even though
no other client was manipulating the directories during the test runs.
(I cannot say for sure at this time, but I suspect it is because,
without a directory
delegation, it is difficult to determine if another client has updated
the directory
and possibly made a name lookup cache hit  fail.)

I can finally say that I think implementation of delegations is worth
the complexity
and effort.

As you may be aware of, I have been working on a draft to allow clients and
servers to "fine tune" requirements for directory delegations with Jeff Layton.
This is hoped to make them easier to implement.
Please take a look at the draft, found here:
 https://www.ietf.org/archive/id/draft-rmacklem-nfsv4-directory-delegations-03.txt

Thanks, rick